'This is not to say that no one should use these drugs at the same time. It is a risk benefit analysis any time you take a drug and maybe this is something people should consider'- Dr. Eric Sun, Stanford University

To perform the study, researchers at Stanford University and the University of California looked at 315,428 privately insured people aged 18 to 64 who had an opioid prescription and who were continuously enrolled in a health plan with medical and pharmacy benefits.

The people studied were not known drug abusers and had been prescribed these drugs by a physician, said lead author Dr. Eric Sun, assistant professor at the department of anesthesiology, pain and perioperative medicine at Stanford University in California.

The study found rising use of the painkillers in combination with anti-anxiety drugs in the period from 2001 to 2013.

Those who took both drugs at once had a significantly higher risk of death than people using opioids alone, said Sun.

During overdoses, benzodiazepines enhance the respiratory suppression associated with opioids and people stop breathing.

This effect is known in the medical community, and Sun said a "black box" warning issued by the U.S. Food and Drug Administration had drawn attention to the potential effect of combining the drugs.

"Probably most clinicians are aware of the increased risk — they might not be aware of the magnitude," he said.

And doctors don't necessarily know all the prescriptions their patients may be taking.

"One doctor might not know what another doctor is prescribing. At Stanford we're all one health system, and I can see what other doctors at Stanford are prescribing, but doctors outside of Stanford, unless you tell me, I don't know." he told CBC News.

He said there might be a role for insurance companies that cover drug prescriptions to alert physicians to dangerous drug combinations.

Opioid users who used benzodiazepines concurrently were at substantially higher risk of an emergency room visit or hospital inpatient admission for opioid-related poisoning or an opioid-related adverse event, the study found.

How to reduce risk

Chronic painkiller users who also took anti-anxiety drugs experienced the highest adjusted incidence of emergency room visits or inpatient admissions.

The researchers estimate elimination of the concurrent use of anti-anxiety drugs and opioid painkillers could reduce the population risk of an emergency room visit for opioid overdose by 15 per cent.

The study's authors urge caution in prescribing opioids concurrently with anti-anxiety drugs, and call for education programs that warn prescribers and patients about the risks of taking both medications in combination.

"This is not to say that no one should use these drugs at the same time. It is a risk-benefit analysis any time you take a drug and maybe this is something people should consider," Sun said.